In urban planning and building design it is necessary to take into account the trajectories of the sun in order to achieve adequate and appropriate planning of urban expansion, and particularly to consider the location, relative to vegetation and nearby buildings, of the individual buildings on the ground, their shape, as well as the position and size of the elements involved in the temperature control process of the building (perimeter walls, outside openings, roof, balconies, projections, etc.). Especially, since it has been realized that conventional energy sources are exhaustible, and their use is more expensive, it is essential to be able to make use of maximum insolation during cold periods and to obtain protection by means of adequate shading during hot periods.
Furthermore, in the study and applications of solar energy, in order to obtain maximum efficiency, it is necessary to know the position of the sun at any time, to orient the parabolic collectors, as well as the optimum inclination in a predetermined period, to position the solar panels.
It is also desirable to relate the data supplied to the times indicated by the clock, in order to facilitate practical operations and to perform any checks.
The major astronomical institutes publish tables which show the angles of position of the sun as a function of latitude, solar declination and time angle; the table for 40.degree. North latitude is published in "L'Architettura dell'evoluzione" (The Architecture of Evolution), by S. Los and N. Pulitzer, Ediz. Luigi Parma, Bologna, p. 446. It is possible to calculate analytically, again as a function of latitude, solar declination and time, the altitude angle and the azimuth angle of the sun. The procedure is described in "L'energia solare nella costruzione" (Solar energy in building), by C. Chauliagnet, Ediz. C.E.L.I., Bologna, p. 26-27. There are also solar charts which show the course of the sun, on a given day and for a given latitude. "L'enciclopedia pratica per progettare e costruire" (The practical encyclopedia for design and building), by E. Neufert--Editore Hoepli, Milan, p. 123-124, shows the solar charts for Rome on the solstice and equinox days; "but (writes Neufert)--by changing the day, as well as by changing the latitude, the course of the sun changes, and, consequently, the solar charts must be different".
"L'energia solare nelle applicazioni termiche"(Solar energy is thermal applications), by J. A. Duffie and W. A. Beckmann, Liguori Editore, p. 64, also shows a nomogram to determine, by a series of successive readings, the time of sunset and the length of the day.
Because of the difficulty of their application, lack of data generally, or lack of specific data particularly, these methods do not fulfill the requirements of the user, who is forced to resort to several tables of information, or to obtain analytically the necessary data, especially when they are related to observation points of different latitude and longitude.